The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. As described in detail in U.S. Pat. Nos. 5,078,168 and 5,221,194, both owned by the assignee of this invention, the recent trend has been to shift away from the use of solvent based coating materials, e.g. varnishes, lacquers, enamels and the like, in favor of water-based coatings which reduce problems of explosiveness and toxicity associated with solvent-based coatings. Unfortunately, this switch from electrostatically spraying solvent-based coating to those of the water-based type has sharply increased the risk of electrical shock among system operators.
The problem of electrical shock from water-based coatings is addressed in U.S. Pat. Nos. 5,078,168 and 5,221,194 wherein a "voltage block" system is provided for transferring electrically conductive coating materials without the formation of a completed electrical path between the source of coating material and a high voltage electrostatic power supply. The system in U.S. Pat. No. 5,078,168, for example, comprises first and second shuttle devices which are serially connected to two large reservoir piston pumps. The first shuttle is movable between a transfer position and a neutral position with respect to a filling station which is connected to a source of electrically conductive coating material. At the filling station, the first shuttle is operative to transfer coating material from the source into the reservoir of the first pump. In the neutral position, the first shuttle is electrically isolated, i.e. physically spaced, from the filling station. The second shuttle device is movable between a transfer position wherein it interconnects the first piston pump with the second piston pump, and a neutral position wherein the two pumps are electrically isolated from one another and the second piston pump supplies coating material to one or more dispensers. Movement of the shuttles is controlled to maintain one of the shuttles in a neutral position at all times during a coating operation so that there is never a completed electrical path between the source(s) of electrically conductive coating material and the electrostatically charged coating material at the dispenser(s).
Systems of the type disclosed in U.S. Pat. Nos. 5,078,168 and 5,221,194, in which the shuttles and pumps are connected in series with one another between the source of coating material and one or more coating dispensers, have suffered to some extent from problems of inadequate pressure at the coating dispensers and pressure fluctuations within the system. These problems have been addressed in U.S. patent application Ser. No. 07/961,156, filed Oct. 15, 1992, now U.S. Pat. No. 5,326,031, entitled "Apparatus for Dispensing Conductive Coating Materials Including Color Changing Capability," which is owned by the assignee of this invention. In this system, electrically conductive coating material is transmitted from two "parallel" flow paths to one or more coating dispensers. Each flow path comprises a voltage block construction including a transfer unit having a filling station connected to the source(s) of coating material, a discharge station spaced from the filling station and a shuttle movable between and releasably coupled to the filling station and to the discharge station. Upon movement of the shuttle to the filling station of the transfer unit within one of the two flow paths, the shuttle is effective to transfer coating material from the source into the reservoir of a piston pump associated with such flow path. When the reservoir of the piston pump is filled, the shuttle moves and is coupled to the discharge station wherein a connection is made allowing the coating material to be transferred from the pump reservoir, through the discharge station of the transfer unit, and, into a valve connected to the dispensers. This valve is common to both flow paths and is effective to switch the flow of coating material to the dispensers from one flow path to the other. The operation of the system is synchronized such that when the pump of one flow path is supplying coating material to the dispensers, the pump of the other flow path is receiving coating material from the source. A voltage block is continuously maintained between the source and charged dispensers, and the dispensers can be essentially continuously supplied with coating material from one or the other of the parallel flow paths.
Both the series system and parallel system described above employ large reservoir piston pumps and connective lines or tubing to transfer the coating material to separate shuttle devices, which, in turn, communicate with the coating dispensers. While these arrangements provide an effective voltage block or discontinuous path between the source(s) of coating material and a coating dispenser(s), there are certain applications in which space considerations and color change requirements make the use of such systems impractical. For example, in automotive or other vehicle paint lines, the coating supply system must be capable of rapidly changing from one color of coating material to another, and there is a limited amount of floor space which the coating system can occupy within the production facility. The provision of separate reservoir pumps and shuttles in both of the systems described above, and the tubing required to interconnect such elements with the source(s) of coating material and coating dispensers, requires more space than is available at many automotive facilities. Additionally, switch-over from a coating material of one color to another is delayed by the presence of separate pumps and shuttles, as well as the tubing therebetween, and the control functions required to achieve such changeover can be complicated.